Fix Banzai reference for milestones belonging to parent groups

lib/banzai/filter/milestone_reference_filter.rb

@@ -65,7 +65,7 @@ module Banzai
         # We don't support IID lookups for group milestones, because IIDs can
         # clash between group and project milestones.
         if project.group && !params[:iid]
-          finder_params[:group_ids] = [project.group.id]
+          finder_params[:group_ids] = project.group.self_and_ancestors.select(:id)
         end
 
         MilestonesFinder.new(finder_params).find_by(params)

spec/lib/banzai/filter/milestone_reference_filter_spec.rb

@@ -3,7 +3,8 @@ require 'spec_helper'
 describe Banzai::Filter::MilestoneReferenceFilter do
   include FilterSpecHelper
 
-  let(:group) { create(:group, :public) }
+  let(:parent_group) { create(:group, :public) }
+  let(:group) { create(:group, :public, parent: parent_group) }
   let(:project) { create(:project, :public, group: group) }
 
   it 'requires project context' do
@@ -340,6 +341,13 @@ describe Banzai::Filter::MilestoneReferenceFilter do
 
       expect(doc.css('a')).to be_empty
     end
+
+    it 'supports parent group references', :nested_groups do
+      milestone.update!(group: parent_group)
+
+      doc = reference_filter("See #{reference}")
+      expect(doc.css('a').first.text).to eq(milestone.name)
+    end
   end
 
   context 'group context' do